The present invention relates to a positioner having a communication function.
Generally in a control system operating a control apparatus of valves by remote control, detection signals from field apparatuses measuring a quantity of physics such as flow quantity, pressure force and temperature are gathered to a controller as a superior apparatus, the controller operates opening and shutting of valves by remote control based on those detection signals. In the control system, generally a communication cable of 4-20 mA is used as a transmission channel transmitting the detection signal from a detection port of field apparatuses to the controller and transmitting a control signal from the controller to an operation port of valves. The controller receives an analog signal (following, “4-20 mA signal”) with an electric current of 4-20 mA normalized to 0-100% from the detection port by this communication cable. Further it establishes a PID parameter so that a detection data in the detection port will become a predetermined target value (set point) at every operation port. And it sends the 4-20 mA signal as the control signal normalized to 0-100% towards the operation port.
In late years, a field apparatus including a function transmitting diagnosis information of a valve and itself to the controller in addition to the control function of the valve has been introduced. As an example of such a field apparatus, there is a positioner disclosed by Japanese Patent Laid-Open No. 1-141202. According to this, because the diagnosis result of valve and itself are transmitted to the controller via a transmission channel, the controller can analyze the diagnosis result and take corresponding action. Therefore a bidirectional digital communication of field bus communications (hereinafter “FB communication”) is used and the control system having the bidirectional digital communication replaces the control system of the conventional analog communication. In such a digital control system, in addition to operating by remote control the operation port of valves as before, the controller instructs the diagnosis of the valve and a diagnosis of the field apparatus itself by remote control and it manages each field apparatus by acquiring the diagnosis information. In the control system having the FB communication described, it is an advantage that setting the maintenance of the valve and the field apparatus are easy.
However, it is needed to exchange the interface of all apparatuses to the interface for FB communication from the existing interface for 4-20 mA communication in order to change the existing control system using the communication cable of 4-20 mA to the control system by the FB communication. Moreover, very many costs are needed in a change of such the system.
Moreover, in the control system using the FB communication, because the control information that operates by remote control of the operation port and the diagnosis information of the apparatus intermingle, the inside of the transmission channel is crowded by the transmission information and interference may influence the control.